Polyphosphonates are known to exhibit excellent fire resistance and it is known (see e.g., U.S. Pat. No. 2,682,522) that linear polyphosphonates can be produced by melt condensing a phosphonic acid diaryl ester and a bisphenol using a metal catalyst (e.g., sodium phenolate) at high temperature. This approach produced low molecular weight polyphosphonates that exhibited poor toughness.
Consequently, to improve toughness a synthetic approach to produce branched polyphosphonates by the transesterification process was developed (see e.g., U.S. Pat. No. 4,331,614). This approach involved the transesterification reaction of a phosphonic acid diaryl ester, a bisphenol, a branching agent (tri or tetra phenol or phosphonic acid ester), and a preferred catalyst (e.g., sodium phenolate) carried out in the melt, usually in an autoclave. Several patents have addressed the use of branching agents in polyphosphonates (see e.g., U.S. Pat. Nos. 2,716,101; 3,326,852; 4,328,174; 4,331,614; 4,374,971; 4,415,719; 5,216,113; 5,334,692; and 4,374,971). These approaches produced branched polyphosphonates, however their cost and properties were not sufficient for general acceptance in the marketplace. For example in branched polyphosphonates (see e.g., U.S. Pat. No. 4,331,614), the number average molecular weights as high as 200,000 g/mole are claimed with a minimum requirement of 11,000 g/mole with polymer dispersities less than 2.5. Consequently these polyphosphonates exhibited high melt viscosities. This approach was successful in producing high molecular weight polyphosphonates that exhibited improved toughness, but processability was sacrificed. Another disadvantage for this process is that it requires high purity monomers, preferably greater than 99.7% that make it expensive. Another shortcoming of both the linear and branched polyphosphonates was the lack of hydrolytic stability, some haze, and in the case of less expensive high volume bisphenols like bisphenol A, limited and insufficient heat stability as determined by Tg.
In summary, linear polyphosphonates produced by the transesterification process exhibited excellent flame resistance and good melt flow (e.g., good processability), but were brittle due to low molecular weight. Branched polyphosphonates produced by the transesterification process exhibited excellent flame resistance and good toughness, but were not easily melt processable due to high molecular weight and low polydispersity. For polyphosphonates made from less expensive high volume bisphenols like bisphenol A, the polyphosphonate had poor thermal and hydrolytic stability.
Quaternary phosphonium catalysts and aqueous-phenol based quaternary phosphonium catalyst mixtures have reportedly been used in the synthesis of polycarbonates (see e.g., U.S. Pat. Nos. 3,442,854 and. 6,291,630B1). However, these catalysts have not been applied to the synthesis of polyphosphonates, nor is it obvious that they would work better than any other known or preferred catalysts for these materials.